1. Field of the Invention
The present invention relates to a shielding sheet for shielding magnetism and a radio wave, and absorbing a radio wave, and the production method of the same.
2. Description of the Related Art
In recent years, computers are used in a wide variety of fields due to the development of communication and information technology, and the systematization of the network has progressed, in which computers are connected through a digital circuit. In the manufacturing industry, many sorts of controlling machines, measuring instruments, sensors, etc. are used in system, resulting from the factory automatization through computers.
However, it comes into question that radio waves of various wave length generated from conductive cables or motors around the computers or various machines and instruments, or illegal radios, form noise causing troubles of computer malfunction or communication failure.
It is also regarded as a matter in question that since the magnetism (alternating magnetic field) from electric cables and motors and so on for power sources leaks to the surroundings, electronic circuits and controlling devices in various equipment cause failures and stoppages. In order to prevent the leakage of the radio waves and magnetism (it will be called an electromagnetic wave, hereafter) or prevent the influence thereof, various shielding materials are proposed.
For instance, as shown in FIG. 8 (A), an electromagnetic wave shielding substance 50 which is composed of an electromagnetic wave shielding layer 52 in which a soft magnetic powder 53 is uniformly dispersed in layers, held between resin sheets 56, 56 with an adhesive agent 54, 54, or, as shown in FIG. 8 (B), another electromagnetic wave shielding substance 60 which is composed of an electromagnetic wave shielding layer 62 in which a flat shaped soft magnetic powder 64 is mixed and uniformly dispersed in a resin 65, covered on both surfaces with a resin film 66, have been proposed (refer to Japanese Patent Laid-open Hei 8-31237).
Since such electromagnetic wave shielding substances 50, 60 have pliability, the shielding substances 50, 60 are wound as a shielding material around an electric cable (not shown) with joining means, such as a slide fastener, along both margins. This method can shield the electromagnetic wave leaked from the electric cable, and can shield interference by the electromagnetic wave from the outside, and can be applied at the working site. (refer to the Patent Bulletin cited above).
However, since the above shielding substance 50 uses the electromagnetic wave shielding layer 52, in which the soft magnetic powder 53 itself is uniformly dispersed in a layer, the shielding substance is heavy and inconvenient to install at the working site, though it looks to have an excellent shieldability. And it has a disadvantage that since both surfaces of the shielding layer 52 are stacked only with flexible resin sheets 56 and adhesives 54, when the shielding substance is wound around the electric cable in a circle, the above shielding layer 52 is apt to collapse in the inside, so that the thickness is changed and the shieldability becomes uneven depending upon the position.
On the other hand, since the shielding substance 60 uses the electromagnetic wave shielding layer 62 in which the flat shaped soft magnetic powder 64 is mixed and uniformly dispersed in the resin 65, the shieldability is uniform and the shielding substance 60 can be easily installed at the working site. However, since the above powder 64 is dispersed all over the resin 65, the shieldability is lowered. When the powder 64 is dispersed in a large quantity in the resin 65 to prevent the lowering, it causes a disadvantage that the pliability of the resin sheet is impaired, resulting in gaining in weight though the shieldability is increased, and the installation at the working site becomes difficult.
An object of the present invention is to provide a shielding sheet which overcomes the disadvantages held by the conventional art, having a high, stable shieldability against the electromagnetic wave, keeping a sufficient pliability all around the sheet, and is able to easily carry out the rolling up process to, for instance, an electric cable, and the method of production thereof